A prior art system for connecting peripheral cards via a bus is illustrated in FIG. 1, which shows three peripheral card labeled 1, 2 and 3, respectively. Each of the cards is connected by a connector pair 4, 5 and 6, respectively, to a bus 7 via an associated stub 8. Bus 7 is, in essence, a parallel cable running over top of the peripheral cards 1 through 3.
It will be noted from FIG. 1 that only the end peripheral cards, e.g., cards 1 and 3, are provided with terminations, e.g., terminating impedances, which are respectively denoted R.sub.1 and R.sub.3. The center card 2, on the other hand, does not include a dedicated termination.
It will also be appreciated that close proximity between the peripheral cards 1 through 3 requires that the bus 7 must be removed from at least two of the cards, i.e., cards 1 and 2 or cards 2 and 3, in order to remove card 2 for maintenance or replacement.
U.S. Pat. No. 3,800,097 discloses a bus connection system wherein selectively installed termination cards and jumper cable cards are used. An empty spot on each bus is connected to a terminator card. However, the system taught by U.S. Pat. No. 3,800,097 is connectable to a plurality of peripheral card types via bus interface units but the overall termination of a chain of cards connected to bus interface unit is not considered.
U.S. Pat. No. 4,658,333 discloses a connector system where the user is allowed to selectively choose whether terminal cards or jumper cables, e.g., non-terminating cards, will be used. This allows the user to easily reconfigure arrangement, e.g., the length, of the bus.
U.S. Pat. No. 4,736,409 discloses the use of cards which can be designated as line or trunk cards. Thus, the 4,736,409 patent teaches the user selection of the bus configuration.
U.S. Pat. No. 5,220,211 discloses a high speed bus transceiver with fault tolerant design for hot pluggable applications, which uses a low impedance differential bus and a plurality of transceivers connected to the bus. Each transceiver is provided with a driver circuit which places data onto the bus and receiver for accepting data from the bus. The driver includes a pseudo-differential current driving circuit arrangement, which sinks current from only one side of the bus while the other side of the differential bus is undisturbed. The receiver includes a differential comparator biased to a preferred output voltage level.
Parallel bus connections are susceptible to a plurality of problems. For example, software and hardware faults can easily result in simultaneous activation of multiple contending device. Moreover, improper signal termination produces a host of problems, primarily noise.
Poorly or inadequately terminated busses are a source of noise in prior art bus systems. In particular, the prior art busses are terminated at both ends with resistors whose value equals the characteristic impedance of the bus. This type of termination is referred to as "odd-mode" termination. This type of termination leads to a situation where "common-mode" signals induced in the bus "ring" for excessive periods of time, causing problems with signal integrity, EMI and common-mode range.
In addition to the above-mentioned noise problem, excessive noise or inadequate noise margin can lead to performance problems through data integrity problems or through a need to reduce the clock rate to avoid data integrity problems. There are potentially several types of noise in a bus system including coupled noise, switching noise, EMI/EMC noise and reflection noise. Coupled noise occurs when a signal on a conductor in the bus induces an undesired signal on a neighboring conductor on the bus. Switching noise occurs when current pulses resulting from multiple bits changing states simultaneously can induce undesired signals on power and/or ground which can, in turn, induce erroneous data signals on driver or receiver outputs. EMI/EMC noise occurs when the bus system either radiates unacceptably in certain frequency bands or is unacceptably sensitive to ambient electromagnetic radiation. Reflection noise occurs when reflections arising from impedance mismatches on the bus distort the data signals at the inputs to the receivers. Impedance mismatches can occur at card slots, as well as at the ends of the bus, due to improper termination.
Moreover, the conventional connecting system provide a potential for damaging peripheral cards when these cards are either connected or disconnected to the system while it is in operation.
The present invention was motivated by desire to overcome the problems with respect to the prior art.